SALT LAKE CITY - Humans and chimpanzees share the ability to
taste, or not taste, a bitter synthetic compound called PTC-as well
as numerous other toxic substancesbut contrary to longstanding
scientific thought, they developed that ability through separate
genetic mutations, according to new research led by University of
Utah and University of Washington geneticists Stephen Wooding,
Ph.D., and Michael Bamshad, M.D.

The ability to taste PTC (phenylthiocarbamide) and other bitter
compounds probably evolved as a way to protect animals from eating
poisonous plants, according to Wooding and Bamshad, senior authors
of a study reported in the April 13 cover story in Nature. Being a
PTC taster or non-taster has far-reaching implications for human
behavior, such as in the foods people eat and even whether they
smoke cigarettes, said Wooding, research assistant professor of
human genetics at the U of U's Eccles Institute of Human
Genetics.

Evidence that humans and chimpanzees harbor different kinds of
mutations suggests that the similar patterns of PTC sensitivity in
the two species may be an evolutionary coincidence. "With this
particular gene, it shows that humans and chimps probably faced
different pressures from natural selection," Wooding said. "Chimps
and humans outwardly show the same patterns of variation, but the
mutations in each species are affecting taste sensitivity in
completely different ways."

About 75 percent of people worldwide can taste PTC, while the
remaining quarter can't. PTC tasters are less likely to smoke
cigarettes than non-tasters, but they're also less likely to eat
cruciferous vegetables, such as broccoli, that are important
sources of nutrition, Wooding said.

Differences in PTC sensitivity were first discovered in 1930,
when American chemist Arthur Fox accidentally let loose some of the
compound in his laboratory. Fox noted that while some people
complained of a bad taste from PTC, others could not taste it all.
After that, he conducted experiments and found that about 70
percent of people were PTC tasters.

Following a study of chimpanzees more than 65 years ago,
scientists thought the gene responsible for the ability to taste
PTC evolved long before humans and chimps diverged into separate
species. But after examining patterns of variation at a molecular
level, Wooding and Bamshad, along with collaborators at the German
Institute for Human Nutrition, Arizona State University, and the
Southwest Foundation for Biomedical Research, concluded that the
"insensitive" form of the gene evolved separately in humans and
chimps through completely different mutations.

People carry two functional forms of the PTC gene, one of which
evolved over the last 1.5 million years or so-long after people and
chimps had emerged as different species 7 to 10 million years ago.
Although they're not sure why humans developed mutations of the
gene that aren't sensitive to PTC, Wooding, Bamshad, and their
co-researchers theorize that people who can't taste the substance
developed sensitivity to a different, bitter compound, which might
have provided an evolutionary advantage.

The researchers also conducted a taste test with 39 chimpanzees,
to observe their sensitivity to PTC. Using a test similar to that
used in many classrooms, in which children are asked to taste a
piece of paper containing minute concentrations of PTC, they fed
plain apples and apples soaked in small concentrations of PTC to
the chimps. (PTC is not harmful and the chimps suffered no ill
effects from eating it.) These results verified the same gene is
responsible for the ability to taste or not taste PTC in both
humans and chimps.

Comparisons of the taste test results with a gene sequence
analysis showed that, like humans, chimps carry two forms of the
PTC gene, but unlike the functional non-taster form that reduces
PTC sensitivity humans, the one found in chimps is broken. After
examining the molecular structure of the gene, Wooding and his
fellow researchers pinpointed a mutation in a single nucleotide-one
of the structural units of DNA-as the difference between the taster
and non-taster forms found in chimps. A single nucleotide's
difference indicates the mutation occurred long after humans and
chimps diverged, Wooding said.

Evidence that chimpanzees have a "broken" form of the gene
indicates that taste sensitivity likely was diminished in chimps
for different reasons than in humans. Wooding and Bamshad theorize
chimps lost some PTC sensitivity as a result of plants disappearing
and their diets being altered over eons, so that they were no
longer pressured to taste the poisons found in those plants. But it
also could be from a change in chimp behavior, such as a change in
diet choice, Wooding said.

"Different mutations in humans and chimps might have emerged for
different reasons," he said, "but in both cases we think what's
driving it is the need to avoid toxins found in plants, which are
common in primate diets."

The PTC gene is one of about 30 known to produce sensitivity to
bitter tastes. A national group of U.S. researchers, including Mark
F. Leppert, Ph.D., professor and co-chair of the U's Department of
Human Genetics and Hillary Coon, Ph.D, research associate professor
of psychiatry, discovered the PTC gene in 2002. If scientists can
unravel the nature of sensitivity to PTC and other bitter
chemicals, they may be able to produce compounds with important
ramifications for human nutrition and diet, according to Wooding
and Bamshad.

"Ultimately, we'd like to connect the patterns of variation in
genes to human behavior," Wooding said.